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MECHANICAL SEPARATION                             263
blinding either rakes traveling back and forth in contact with the screen are used as is illustrated in Fig. 1 or brushes, see Fig. 2. Figure 2 shows a double-threaded screw for actuating the brushes and reversing the direction of travel at the ends of the screens. Punched steel plates are used for screens on the separators.
The wheat first goes to a receiving or scalping separator with large screen holes, % and M in-, the oversize from this screening operation yielding all sorts of field refuse, coils of fibers, twigs, stones, bits of coal, etc., which go to the boilers as fuel. The undersize or partially cleaned wheat then passes to a separator with K-in. holes for the first separation, yielding wheat, which fails to pass these holes or passes them but slightly and undersize material consisting of broken wheat or small seeds or small grains which is ground for stock feed. The material freed from this small stuff passes to screens on the same frames with J{ e-i&- holes, the wheat passing through and the oversize consisting largely of oats and corn goes to a special separator to be further separated for stock feed. In the mill where the particular separation means being described are used, the upper section of each screen of the No. 2 separator is 10 in. wide, the lower section with its K Q-m. holes being 14 in. wide. Considerable variations in ,the separatory practice exist in different mills.
The special separator for the oats-corn mixture is a single two-high machine, upper screen has %-in. holes.    The oversize of this machine is refuse.    The acree. below on to which the undersize of the upper discharges has K6~m- holes.    Tht oversize is a corn product.    The undersize is mostly oats.
The wheat leaving the %6-in. screen of the No. 2 separator undergoes scouring, washing and other conditioning operations following which it again passes through separators to remove sand, bran, etc., before undergoing the first rolling operation.
Cockle Cylinders.—It seems proper at this point to describe the principle of separation involved in the cockle cylinder since these devices are employed in flour mills and grain elevators in conjunction with separators of the kind already described or stationary screens of the needle-slot kind. Cockle is a generic term for small weed seed.
The cockle cylinder has on its interior numerous and closely spaced indentations. If the intent is to separate wheat from small weed seed the indentations are made just large enough to receive the weed seed but not the wheat. The cylinder is inclined and it revolves, the progress of the wheat is consequently the same as in a trommel. The weed seeds in the depressions are carried around by the revolution of the cylinder and fall out by gravity into a fixed trough which runs through the length of the cylinder. This trough is provided with a spiral conveyor for carrying off the weed seed. By varying the size and shape of the indentations other separations can be effected. These devices have been developed in the grain business but they or the principle involved have application outside of it. The capacity of cockle cylinders ranges from 20 to 25 bu. per hour. The diameter ranges from 20 to 28 in. and the length from 7 to 11 ft.
Air Separation.—The separation of fine material from coarse by bringing a stream of material to be separated under the suction of a fan has been mentioned in describing the grain separators. The means for separating the solid particles from the air will be described later. Conical separators are much used for this purpose and in grinding and separating systems such as those of the Raymond Brothers impact pulverizer the grinding and separation are done in close circuit. The ground product is aspirated into u cone the coarse falling back into the grinding machine. The fine product which fails to fall back is collected in a centrifugal cone and is removed through a gate from time to time. The air from the second